In this paper, we propose a novel copper balls interconnection technology for 60 GHz band 3-D system-in-package (SiP) modules for 10mm square 60 GHz band ultra-small wireless personal area network (WPAN) using organic substrates. Copper ball diameter of 350 μm is used for interconnection between the substrates. Ground copper balls placement is discussed showing electric field distribution. As a result, six ground copper balls around vertical signal line interconnection part are crucial for improvement of transmission characteristics. The six ground copper balls interconnection structure have similar results up to about 70GHz for S21 compared with through CPW, hence copper balls interconnection technology is usable up to about 70 GHz.